1. Field of the Invention
The present invention relates to a flexible tube of an endoscope, and particularly, to a durable and heat resistant covering layer of the flexible tube.
2. Description of the Prior Art
An endoscope is a medical apparatus inserted into a patient's body cavity. The endoscope has a photographing unit to pick up images inside the patient. The images are displayed on a display and are used for diagnosis.
FIG. 1 shows an example of an endoscope. The endoscope has a controller 1 and an inserter 4 to be inserted into a patient's body cavity. The inserter 4 has a flexible tube 2 and a bending portion 3. The angle of the bending portion 3 is adjustable through the controller 1.
FIG. 2 is a longitudinal section showing the flexible tube 2. The flexible tube 2 has a spiral metal flex 8, a braid 7 of stainless steel wires wound around the flex 8, and a covering layer 6 covering the braid 7. The covering layer 6 is a lamination of layers of rubber and urethane-based thermoplastic elastomer.
The flexible tube 2 must be flexible to enter a patient's body cavity and must be hard to transfer insertion force to the leading end thereof. To provide the flexibility and bending rigidity to the flexible tube 2, a conventional endoscope forms the covering layer 6 with a lamination of layers of dissimilar materials. Due to the dissimilarity, the layers insufficiently adhere to one another and peel off after extended use of the endoscope, thereby deteriorating the insertion ability and appearance of the endoscope.
When the covering layer 6 is partly formed from the urethane-based thermoplastic elastomer, it demonstrates poor heat resistance and softens when it is used at high temperatures or after high-temperature sterilization. This results in deteriorating the insertion ability of the endoscope. In addition, the urethane-based thermoplastic elastomer causes hydrolysis and reduces the hardness of the flexible tube 2.
The urethane-based thermoplastic elastomer is too hard to form a soft part. Even if the soft part is made from the elastomer, the soft part must be laminated with a hard layer to provide bending rigidity. Such lamination complicates production processes and weakens durability in interlayer areas.
Alternatively, the flexible tube 2 may be made from hard segments of polybutylene terephthalate (PBT) and soft segments of polyester-based thermoplastic elastomer having aliphatic polyester or aliphatic polyether. These materials, however, provide insufficient softness and raise the same problem as the urethane-based thermoplastic elastomer.